Use of an austenitic stainless steel

ABSTRACT

Austenitic alloy with following composition, in weight-percent: Cr 23-30, Ni 25-35, Mo 2.0-6.0, Mn 1.0-6.0, N 0-0.4, C up to 0.05, Si up to 1.0, S up to 0.02, Cu up to 3.0, W 0-6.0, one or more element of Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0 and balance Fe and normally occurring impurities and additions as wire in oil- and gasextraction, specially reinforcement wire in the application wirelines and which shows a combination of good corrosion resistance and good mechanical properties, especially a tensile strength of at least 310 kpsi at diameters of 1.0 mm and thinner.

TECHNICAL FIELD

The present invention relates to an austenitic stainless steel alloy with high Cr-, Mo-, Mn-, N- and Ni-contents with a combination of high corrosion resistance and good mechanical properties, such as high tensile strength and good ductility, which is especially suitable for use for applications in the extraction of oil and gas, such as e.g. wire, more specially as reinforcement wire in the application wirelines.

BACKGROUND

In connection with that accesses to natural resources as e.g. oil and gas become more and more limited, the deposits smaller and of lower quality, one tries to find new deposits or such that until now were never exploited due to much too high costs for extraction and subsequent processes, as e.g. transport and refinement of the raw material, the maintenance of the source and measuring work cycles.

The extraction of oil and gas from the bottom of the sea in very deep water is a conventional technology. The transports of as well equipment as goods to and from the source as well as energy- and signal transfer will be managed from the surface of the sea. In very deep water it can be a question of transport distances of up to 10.000 meter for these applications.

To an ever increasing extend so-called wirelines or well logging cables of stainless steel, i.e. ropes or strands are used within applications for offshore oil-and gasextraction. Nowadays, these are usually designed in such way that they contain a plurality of insulated electric leaders or cables, such as e.g. fiber optical cables, which are in their entirety covered of one or more bearing helical wound steel wire.

GB-A-2329722, which hereby is included as reference gives an overview about the state of the art regarding wellbore logging and forms of execution for well logging cables, without claiming to be complete.

The selection of steel grades is primarily determined by the demand on strength, tensile strength, ductility and corrosion properties, especially under the prevailing conditions in an oil- or gas well and by temperatures up to 250° C. The application is to a large extent limited by the load-resistances to fatigue due to repeated use in oil and gasindustry, especially in applications as slickline or wellbore logging cable and in this application of repeated reeling and transport over a so-called pulley wheel Further, the possibilities to use the material within this sector are limited by the breaking load of the cable/slickline-wires. Nowadays, using cold-drawn material will maximize the breaking load. Usually the degree of cold-deformation will be optimized with regard to the ductility. Nowadays, for the production of ropes and strands in this application primarily conventional stainless austenitic steel will be used, e.g. of type AISI303, AISI304 or AISI316 according to U.S. Pat. No. 4,214,693, which through this reference is enclosed in the description or that duplex stainless steel, which is marketed under the trademark FERRINOX 255 according to GB-A-2,354,264, which through this reference is enclosed in the description. The requirements on the corrosion resistance, where the above-mentioned steels are used are insignificant, when the strand or cord is covered of a plastic material as e.g. polyurethane. Newer developments have the purpose to reduce the dead weight of the reinforcement wire as external layer.

The corresponding profile of requirements can also be directed to strip- and wiresprings, where high requirements on strength, fatigue- and corrosion properties occur. In all those cases the use is ascending limited by reasons of corrosion or load.

Further, there are existing requests on a significant higher strength than today's technique allows by a given degree of cold-deformation. A strength conducting to that normally occurring wire dimensions does not plasticize on the surface or which enables use of thinner dimensions, i.e. diameters of 1.0 mm and thinner, are desirable.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a stainless steel alloy with austenitic matrix and simultaneously high strength in combination with high ductility and load-resistance against general corrosion at high temperatures, especially at temperatures up to 250° C.

It is another object of the present invention to provide a stainless steel alloy with a tensile strength of at least 310 kpsi at wire diameters of 1.0 mm or thinner in applications reinforcement wire for well logging cable.

It is another object of the present invention to provide a steel alloy with good corrosion resistance in chloride containing- and environments with high general corrosion.

These purposes are fulfilled with an alloy according to the present invention, which contains (in weight-%): Cr 23.0-30.0 Ni 25.0-35.0 Mo  2.0-6.0 Mn  1.0-6.0 N   0-0.4 C up to 0.05 Si up to 1.0 S up to 0.02 Cu up to 3.0 W   0-6.0

One or more element of Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0

and the balance Fe and normally occurring impurities and additions.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows function of strength against temperature under hot-working for the embodiments S and P of the present invention.

FIG. 2 shows function of strength against temperature under hot-working for embodiments X and P of the present invention.

FIG. 3 shows tensile strength as function of the reduction of the cross-section.

FIG. 4 shows the load as function of the length for wire produced of the alloy according to the invention.

FIG. 5 shows the tensile strength of delivery finish.

FIG. 6 shows how large load inclusive dead weight and bending stress a wire produced of the new alloy compared to wire produced of the well-known alloy UNS N08028, can carry as feature of the pulleywheel diameter.

DETAILED DESCRIPTION OF THE INVENTION

A systematic development effort has surprisingly shown that an alloy with an alloying content according to the present invention satisfies these conditions.

The alloy according to the invention contains therefore, in weight-percent: Cr  23-30 Ni  25-35 Mo 2.0-6.0 Mn   1-6 N   0-0.4 C up to 0.05 Si up to 1.0 S up to 0.02 Cu up to 3 W   0-6.0

a or more of the Elements Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0

and the balance Fe and normally occurring impurities and additions.

The importance of the alloying elements for the present alloy is the following:

Nickel

A high content of nickel homogenizes a high alloyed steel by increasing the solubility of Cr and Mo. Thereby the austenite stabilizing nickel suppresses the forming of the unwanted phases sigma-, laves- and chi-phase, which to a large extent consists of exactly the alloying elements chromium and molybdenum. However, a disadvantage is that nickel decreases the solubility in the alloy and detoriates the hot workability, which entails an upper limitation for the nickel quantity in the alloy. However, the present invention has shown that high contents of nitrogen can be allowed at nickel contents according to the above-mentioned by balance the high nickel content to high chromium- and manganese-contents.

The Ni-content of the alloy should therefore be limited to 25.0-32.0 weight-%, preferably to at least 26.0 weight-%, more preferably at least 30.0 weight-% or 31.0 weight-%. The upper limit for the Ni-content is preferably 34.0 weight-%

Chromium (Cr) is a very active element in order to improve the resistance to a plurality of corrosion types. Furthermore, a high chromium-content implies that one may a very good N-solubility in the material. It is thus desirable to keep the chromium-content as high as possible in order to improve the corrosion resistance. For very good amounts of the corrosion resistance, the chromium-content should be at least 24.0 weight-%, preferably 27.0-29.0 weight-%. However, high contents of Cr increase the risk for intermetallic precipitations, for what reason the chromium-content must be limited upwards to max 30.0 weight-%

Molybdenum (Mo) In modern corrosion resistant austenitic steels frequently a high alloying addition of the element molybdenum will be made in order to increase the resistance to corrosion attacks in general.

In difference to chromium, molybdenum elevates the corrosion rate. An explanation is molybdenum's tendency of precipitation, which gives rise to unwanted phases during sensitization Therefore, a high chromium-content is chosen in favor of a high molybdenum content, even in order to obtain an optimum structural stability of the alloy. Certainly, both alloying elements increase the tendency of precipitation, but tests show that molybdenum does this more than twice as much as chromium. In the present it is possible to wholly or partly replace the content of molybdenum with tungsten. However, preferably at least 2.0 weight-% of molybdenum shall be added to the alloy. Therefore the molybdenum content should be limited to between 2.0 and up to 6.0 weight-%, preferably to at least 3.7 weight-%, preferably to at least 4.0 weight-%. The upper limit for molybdenum content is 6.0 weight-%, preferably 5.5 weight-%.

The precipitation of intermetallic phase is favored by increasing contents of chromium and molybdenum, but can be restrained by addition of N as well as Ni to the alloy. The Ni-content is limited mainly by aspect of costs as well as of that it strongly decreases solubility of N in smelt. Consequently, the N-content is limited of the solubility in smelt and also in the solid phase, where precipitation of Cr-nitrides can occur.

In order to increase the solubility of nitrogen in the smelt, the Mn- and Cr-content can be increased and the Ni-content be decreased. However, Mo is considered giving rise to increased risk for precipitation of intermetallic phase, why it has been considered being necessary to limit this content However, higher contents of alloying elements have not been limited with consideration to the structural stability.

Tungsten (W) increases the load-resistance to pitting- and crevice corrosion. But alloying with too high contents of tungsten in combination with that the Cr-content as well as Mo-content are high, means that the risk for intermetallic precipitations increases. The content of tungsten should therefore lie within the range of 0 to 6.0 weight-%, preferably 0 to 4.0 weight-%.

Manganese (Mn) Manganese is of vital importance for the alloy because of three reasons. For the final product a high strength will be aimed at, for what reason the alloy should be strain hardened by cold working. Both nitrogen and manganese are known for decreasing the energy, which in turn leads to that dislocations in the material dissociate and form Shockley-particles The lower the stacking fault energy the greater the distance between the Shockley-particles and the more aggravated the sideslipping of the dislocations will be, which makes that the material gets tendencies to strain harden. On these grounds, high contents of manganese and nitrogen are very important for the alloy. A rapid strain hardening will be visualized in the reduction graphs, which will be presented in FIG. 2, where the new alloy will be compared with the already known steels UNS N08926 and UNS N08028.

Furthermore, manganese increases the solubility of nitrogen in the smelt, which further speaks in favor of a high content of manganese. Solely the high content of chromium does not make the solubility sufficient since the content of nickel, which decreases the solubility, was chosen higher than the content of chromium.

A third reason for a manganese-content within the range of the present invention is that a yield stress analysis exhibited at elevated temperature surprisingly shows manganese improving the influence on the hot workability of the alloy. The higher alloyed the steels are, the more difficult they are to work and the more important are additions, which increase the hot workability and which both simplify and make the production cheaper. A manganese-addition implies a decreasing of the hardness during hot working, which one can understand from diagram on FIG. 1, which shows the required stress during hot working for variants of the alloy with high respectively low manganese-content. The good hot workability of the alloy makes alloy excellent suitable for the production of tubes, wire and strip etc. However, manganese has been found influencing negatively on hot ductility of the alloy, such as described by the formula below. Its strong positive influence as hardness decreasing alloying element during hot working was estimated as more important.

Therefore, the content of manganese of the alloy should be higher than 2.0 weight-%, preferably 3.0 to 6.0 weight-%, more preferably 4.0-6.0 weight-%.

Carbon (C) has limited solubility in both ferrite and austenite. The limited solubility implies a risk for precipitation of chromium-carbides and therefore the content should not exceed 0.05 weight-%, preferably not exceed 0.03 weight-%.

Silicon (Si) is utilized as desoxidation agent for the steel production and also increases the flowability during production and welding. However, too high contents of Si lead to precipitation of unwanted intermetallic phase, for what reason the content should be limited to max 1.0 weight-%, preferably max 0.8 weight-%, more preferably to 0.5 weight-%.

Sulfur (S) influences the corrosion resistance negatively by forming easily dissolvable sulfides. Furthermore, the hot workability deteriorates for what reason the content of sulfur is limited to max 0.02 weight-%.

Nitrogen (N) is like molybdenum a popular alloying element in modem corrosion resistant austenites in order to increase the corrosion resistance, but also the alloys mechanical strength. For the present alloy it is foremost the increasing of the mechanical strength by nitrogen, which will be exploited. As mentioned above, a powerful increase in strength during cold deformation can be obtained. The invention exploits also that nitrogen increases the mechanical strength of the alloy as consequence of interstitial soluted atoms, which cause stresses in the crystal structure. A high strength is of vital importance for the intended applications as sheets, heat exchangers, production tubes, wire- and strip springs, riggwire, wirelines and also slicklines. By using a high tensile material, the possibility is given to obtain the same strength, but with less material consumption and thereby less weight. Simultaneously, this increases the requirements on the ductility of the material. For springs their susceptibility for absorbing elastic energy is of decisive importance.

Therefore the nitrogen content should be 0.20-0.40 weight-%, preferably 0.35-0.40 weight-%.

Copper (Cu) The influence of copper on corrosion properties of the austenitic steel is disputed. However, it seems to be clear that copper strongly improves the corrosion resistance in sulfuric acid, which is of big importance for range of applications of the alloy. During tests copper has also shown being an element, which is favorable for the production of tubes, for what reason a copper-addition is particularly important for material aimed for tube applications. However, from the experience it is known that a high copper-content in combination with a high manganese-content strongly detonates the hot ductility, for what reason the upper limit for copper-content is determined to 3.0 weight-%. The copper-content is preferably at the most 1.5 weight-%.

At least one of the ductility additions as magnesia (Mg), calcium (Ca), cerium (Ce), boron (B), lanthanum (La), praseodym (Pr), zirconium (Zr), titanium (Ti) and neodymium (Nd) should be added in a content of up to 2.0 weight-% in purpose to improve the hot workability.

In the following some embodiments of the alloy according to the invention are described. They are intended to illustrate the invention, but not to limit it.

EXAMPLES

In the following tables the compositions for the tested alloys according to the invention and for known alloys are specified in comparing purpose. For the known alloys being used as references are, in those cases where they were used for testing, the interval which defines the composition that was tested and which lies within the standard for the alloy. Table 1 shows some embodiments for the alloy according to the invention. TABLE 1 Heat C Si Mn Cr Ni Mo Cu N A 0.009 0.28 5.04 26.4 30.49 5.78 0.025 0.372 B 0.011 0.27 5.10 26.5 33.70 5.90 0.011 0.380 C 0.008 0.27 4.95 26.7 30.77 5.22 0.011 0.357 E 0.01 0.28 4.73 27.2 30.69 4.47 0.011 0.354 I 0.015 0.22 1.03 27.71 34.86 3.97 0.500 0.410 J 0.008 0.33 4.99 26.21 33.37 4.86 1.900 0.370 K 0.007 0.28 5.29 26.25 33.83 3.70 0.011 0.390 L 0.012 0.25 5.18 26.36 33.70 5.10 0.940 0.390 M 0.010 0.25 5.04 27.80 35.40 5.47 2.220 0.400 N 0.008 0.26 3.96 28.98 34.68 5.61 2.250 0.380 O 0.008 0.24 5.70 23.57 24.82 5.00 0.013 0.360 P 0.015 0.24 1.07 26.91 30.77 6.41 1.180 0.220 S 0.015 0.22 5.57 26.11 30.30 6.20 1.150 0.200 T 0.017 0.26 2.97 26.18 30.87 5.86 1.160 0.290 X 0.0147 0.24 1.14 27.72 29.87 3.91 1.480 0.250

Example 1

The tension, which is required during hot working of the present alloy, at different manganese- and molybdenum contents, is shown in FIGS. 1 and 2. The negative influence of molybdenum on the required tension is demonstrated of variants X and P in FIG. 1. The positive influence of manganese on the required tension is demonstrated of variants S and P in FIG. 2.

Example 2

The substantial better increase of the tensile strength by cold working of the present alloys, variants B, C and E, in comparison with the known UNS N08028 and UNS N08926 is shown in FIG. 3.

The ductility of the material in delivery finish was evaluated with the help of businesstypical torsion- and winding-tests. The torsion-test was executed by twisting a 20-cm long wire until that that the breaking arises, but at least 5 turns. The winding-tests were executed by winding a wire at least 5 turns round its own axis and subsequently wind the wire up without that breaking or cracks arise. The present invention manages the requirements on the ductility also in a high strength supply condition. As described in FIG. 4 the alloy of the present invention manages the requirement son the ductility also at amounts of the tensile strength exceeding 310 kpsi.

Example 3

In FIG. 5 and FIG. 6 substantial properties for the wire for the application wirelines are illustrated.

The stress-diagram for a wire in wireline-applications is mainly assembled of three components, which are shown in Table 2: the dead weight of the wire according to equation (1), the carried load according to equation (2) and also of the tension, which is induced of the measurement equipment's support-wheel according to equation (3) and the total tension as the total of the partial stresses according to equation (4). As described by these expressions for the various tensions, a higher yield point in tension/tensile strength allows the use of a smaller feeding-wheel as well as a bigger carried load per cross-section unit. TABLE 2 Case of load Expression for the induced stress (1) Dead weight σ₁ = ρgl/2; ρ = density of the of the wire material, g = acceleration of gravity, l = free length of wire in the bore (2) Carried load σ₂ = F/A; F = carried load, A = cross-section of the wire (3) Pulley wheel σ₃ = dE/R; d = diameter of the wire, E = E-module, R = radius of the support-wheel (4) Total σ = σ₁ ₊ σ₂ ₊ σ₃

Table 2 shows how large the load above the dead weight, wire produced of an alloy according to the invention compared with wire produced of the well-known alloy UNS N08028 can carry as function of the length of the wire.

The density of the alloys was assumed to ρ=8 000 kg/m³.

The acceleration of gravity was approximated to g=9,8 m/s².

A long wire, as in the intended application slickline can be up to approximately 30.000 foot long and gets a noticeable dead weight, which loads the wire. This dead weight is generally carried up of a wheel of different curvature, which gives further rise to loads for the wire. The smaller the radius of the curvature of the wheel the higher the bending stress for the wire gets. Simultaneously, a smaller wire diameter manages stronger curvatures. FIG. 6 shows how large load inclusive dead weight and bending stress a wire, produced of the new alloy compared to wire produced from of the well-known alloy UNS N08028, can carry as function of the diameter of the breaking wheel.

The elasticity module of both alloys was estimated to E=198 000 MPa.

The calculations for the diagram are made under the assumption that the drop of strength is purely linear elastic and the maximum carried load was determined by the yield stress (Rp0.2) of the material.

Example 4

The alloy according to the invention shows surprisingly a very high corrosion resistance in environments, which are relevant for the application wirelines. Until now the test shows surprising results, but is in moment of writing still not brought to an end. The test was exhibited in an environment according to the following:

Saturated NaCl (26 weight-%)+5 weight-% MgCl₂+5% H₂S at 177° C. and 5000 psi (34.5MPa) during 336 hours.

Example 5

A wire with a diameter of 1.0 mm of the steel alloy according to the present invention, and with a composition presented in Table 3, reduced with 69,80%, had an ultimate tensile strength before annealing of 277 kpsi (1910 MPa). The wire was then annealed at 260° C. during 24 hours and showed thereafter a tensile strength of 310 kpsi (2135 MPa).

The material showed also good results in the torsion- and winding tests as described in example 2. TABLE 3 Heat C Si Mn P S Cr Ni Mo Co Cu N Al Pb Bi Nb B Ce Ti R 0.012 0.34 5.04 0.016 0.001 26.44 33.96 5.26 0.025 0.08 0.38 0.021 0.00016 0.00003 <0.01 0.0023 0.01 <0.005 

1. An austenitic stainless alloy comprising the following composition, in weight-percent: Cr  23-30 Ni  25-35 Mo   3-6 Mn   1-6 N 0.0-40 C up to 0.05 Si up to 1.0 S up to 0.02 Cu up to 3.0 W   0-6.0

one or more of the Elements Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0 and the balance Fe and normally occurring impurities and additions.
 2. The austenitic stainless alloy of claim 1, wherein a tensile strength of the austenitic stainless alloy is at least 310 kpsi at a diameter of less than or equal to 1.0 mm.
 3. A reinforcement wire for a wireline having the composition of the austenitic stainless alloy of claim
 1. 4. The reinforcement wire of claim 3, wherein the wireline is in an oil and gas extraction application.
 5. The use of the austenitic stainless alloy of claim 1 in oil and gas extraction. 